Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system, including a Deep Brain Stimulation (DBS) system.
As shown in FIGS. 1A-1C, a SCS system typically includes an Implantable Pulse Generator (IPG) 10 (Implantable Medical Device (IMD) 10 more generally), which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery 14 (FIG. 1C) necessary for the IMD 10 to function, although IMDs can also be powered via external RF energy and without a battery. The IMD 10 is coupled to electrodes 16 via one or more electrode leads 18, such that the electrodes 16 form an electrode array 20. The electrodes 16 are carried on a flexible body 22, which also houses the individual signal wires 24 coupled to each electrode. In the illustrated embodiment, there are eight electrodes (Ex) on each lead 18, although the number of leads and electrodes is application specific and therefore can vary. The leads 18 couple to the IMD 10 using lead connectors 26, which are fixed in a non-conductive header material 28, which can comprise an epoxy for example.
As shown in the cross-section of FIG. 1C, the IMD 10 typically includes a printed circuit board (PCB) 30, along with various electronic components 32 mounted to the PCB 30, some of which are discussed subsequently. Two coils (more generally, antennas) are show in the IMD 10: a telemetry coil 34 used to transmit/receive data to/from an external controller (not shown); and a charging coil 36 for charging or recharging the IMD's battery 14 using an external charger, which is discussed in detail later.
FIG. 2 shows the IMD 10 in communication with an external charger 50 used to wirelessly convey power to the IMD 10, which power can be used to recharge the IMD's battery 14. The transfer of power from the external charger 50 is enabled by a primary charging coil 52. The external charger 50, like the IMD 10, also contains a PCB 54 on which electronic components 56 are placed. Again, some of these electronic components 56 are discussed subsequently. A user interface 58, including touchable buttons and perhaps a display and a speaker, allows a patient or clinician to operate the external charger 50. A battery 60 provides power for the external charger 50, which battery 60 may itself be rechargeable. The external charger 50 can also receive AC power from a wall plug. A hand-holdable housing 62 sized to fit a user's hand contains all of the components.
Power transmission from the external charger 50 to the IMD 10 occurs wirelessly and transcutaneously through a patient's tissue 25, via inductive coupling. FIG. 3 shows details of the circuitry used to implement such functionality. Primary charging coil 52 in the external charger 50 is energized via charging circuit 64 with an AC current, Icharge, to create an AC magnetic charging field 66. This magnetic field 66 induces a current in the secondary charging coil 36 within the IMD 10, providing a voltage across coil 36 that is rectified (38) to DC levels and used to recharge the battery 14, perhaps via a battery charging and protection circuitry 40 as shown. The frequency of the magnetic field 66 can be perhaps 80 kHz or so. When charging the battery 14 in this manner, is it typical that the housing 62 of the external charger 50 touches the patient's tissue 25, perhaps with a charger holding device or the patient's clothing intervening, although this is not strictly necessary.
The IMD 10 can also communicate data back to the external charger 50 during charging using reflected impedance modulation, which is sometimes known in the art as Load Shift Keying (LSK). This involves modulating the impedance of the charging coil 36 with data bits (“LSK data”) provided by the IMD 10's control circuitry 42 to be serially transmitted from the IMD 10 to the external charger 50. For example, and depending on the logic state of a bit to be transmitted, the ends of the coil 36 can be selectively shorted to ground via transistors 44, or a transistor 46 in series with the coil 36 can be selectively open circuited, to modulate the coil 36's impedance. At the external charger 50, an LSK demodulator 68 determines whether a logic ‘0’ or ‘1’ has been transmitted by assessing the magnitude of AC voltage Vcoil that develops across the external charger's coil 52 in response to the charging current Icharge and the transmitted data, which data is then reported to the external charger's control circuitry 72 for analysis. Such back telemetry from the IMD 10 can provide useful data concerning charging to the external charger 50, such as the capacity of the IMD's battery 14, or whether charging of the battery 14 is complete and operation of the external charger 50 and the production of magnetic field 66 can cease. LSK communications are described further for example in U.S. Patent Application Publication 2013/0096652.
External charger 50 can also include one or more thermistors 71, which can be used to report the temperature (expressed as voltage Vtherm) of external charger 50 to its control circuitry 72, which can in turn control production of the magnetic field 66 such that the temperature remains within safe limits. See, e.g., U.S. Pat. No. 8,321,029, describing temperature control in an external charging device.
Vcoil across the external charger's charging coil 52 can also be assessed by alignment circuitry 70 to determine how well the external charger 50 is aligned relative to the IMD 10. This is important, because if the external charger 50 is not well aligned to the IMD 10, the magnetic field 66 produced by the charging coil 52 will not efficiently be received by the charging coil 36 in the IMD 10. Efficiency in power transmission can be quantified as the “coupling” between the transmitting coil 52 and the receiving coil 36 (k, which ranges between 0 and 1), which generally speaking comprises the extent to which power expended at the transmitting coil 52 in the external charger 50 is received at the receiving coil 36 in the IMD 10. It is generally desired that the coupling between coils 52 and 36 be as high as possible: higher coupling results in faster charging of the IMD battery 14 with the least expenditure of power in the external charger 50. Poor coupling is disfavored, as this will require high power drain (e.g., a high Icharge) in the external charger 50 to adequately charge the IMD battery 14. The use of high power depletes the battery 60 in the external charger 50, and more importantly can cause the external charger 50 to heat up, and possibly burn or injure the patient.
Generally speaking, if the external charger 50 is well aligned with the IMD 10, then Vcoil will drop as the charging circuitry 64 provides the charging current Icharge to the charging coil 52. Accordingly, alignment circuitry 70 can compare Vcoil, preferably after it is rectified 76 to a DC voltage, to an alignment threshold, Vt. If Vcoil<Vt, then external charger 50 considers itself to be in good alignment with the underlying IMD 10. If Vcoil>Vt, then the external charger 50 will consider itself to be out of alignment, and can indicate that fact to the patient so that the patient can attempt to move the charger 50 into better alignment. For example, the user interface 58 of the charger 50 can include an alignment indicator 74. The alignment indicator 74 may comprise a speaker (not shown), which can “beep” at the patient when misalignment is detected. Alignment indicator 74 can also or alternatively include one or more Light Emitting Diodes (LED(s); not shown), which may similarly indicate misalignment.
Charger-to-IMD coupling depends on many variables, such as the permeability of the materials used in the external charger 50 and the IMD 10, as well materials inherent in the environment. Coupling is also affected by the relative positions of the external charger 50 and IMD 10, as shown in FIGS. 4A-4C. For best coupling (higher values of k), it is preferred that axes around which coils 52 and 36 are wound (52′ and 36′) are parallel and collinear, with the coils 52 and 36 as close as possible (d1) to each other, as shown in FIG. 4A. Distance d1 indicates the depth between the external charger 50 and the IMD 10, and is generally constant given that the external charger is generally placed on the patient's tissue 25, and that the IMD 10 has been implanted at a particular depth. Deviations from these ideal conditions will generally reduce coupling, as shown in FIGS. 4B-4C. In FIG. 4B for instance, the coil axes 52′ and 36′ are not collinear, but instead are laterally offset (x). In FIG. 4C, the coil axes 52′ and 36′ are parallel and collinear, but the IMD 10 is relatively deep (d2). In any of these non-ideal cases, coupling will be reduced, meaning that the IMD's battery 14 will not charge as quickly, or that the external charger 50 must output more power (e.g., Icharge must be higher) to affect the same charging rate of the IPG's battery 14.
It should be noted with reference to FIG. 4C that the depth d2 of the IMD 10 cannot generally be changed, as this parameter results from how the IMD 10 was implanted in the patient. As a result, the external charger 50 may be in alignment with the IMD 10, even if the coupling between the external charger 50 and the IMD 10 is relatively poor (and thus Vcoil is relatively high). It can be useful to adjust the alignment threshold Vt (i.e., upwards) used by the alignment circuitry 70 in such cases so that the external charger 50 will not unreasonably indicate misalignment to the patient when there is nothing the patient can do to improve alignment. U.S. Pat. No. 9,227,075 describes one technique for adjusting Vt to address alignment as a function of implant depth, although this technique is not described here.